"Mainstream Linux distributions typically default to one of two desktop environments, KDE or GNOME. Both of these environments provide users with an intuitive and attractive desktop, as well as offering a large raft of multimedia software, games, administration programs, network tools, educational applications, utilities, artwork, web development tools and more. However, these two desktops focus more on providing users with a modern computing environment with all the bells and whistles featured in Windows Vista, rather than minimising the amount of system resources they need. For users and developers who want to run an attractive Linux desktop on older hardware, netbooks, or mobile internet devices, neither KDE or GNOME may be a viable option, as they run too slowly on low spec machines (such as less than 256MB RAM and a 1 GHz processor). This article seeks to identify the best lean desktops for Linux, for users that have old or even ancient hardware."

Tom already wrote in the series "Why DE sucks" how GNOME redraws are slow. I have a good 3D card, my video is working wonderfully well and still, GTK+ is slow to redraw. This redraw glitch and Nautilus are one of the greatest GNOME shortcommings. Take that away and Ubuntu will be a real alternative to Windows.

The only desktop for Linux that uses the GPU for accelerated graphics for the desktop is KDE4. Even then, nvidia has had a broken driver for Linux for over 2 years now that is absolutely horrible for 2D graphics ... nvidia is a graphics decelerator on Linux (I'm not sure if it is fixed yet or not).

Anyway, if your system has a reasonable non-nvidia graphics GPU, with working 2D accelerated graphics, then KDE 4.1 and later will be by far and away the fastest desktop for Linux for your system. KDE 4.2 (now in beta) reportedly rocks ... it blows everything else away.

The caveat here is that your system must have accelerated graphics GPU (the requirements for which are not high, just that it works and does actually accelerate the graphics) and enough RAM, at least 256MB but 512MB should probably be considered a practical minimum. This therefore cannot be called a "lightweight" desktop ... but nevertheless, if your system does meet the minimum requirements, as many do, then KDE 4.1+ will be the fastest desktop for your system, and it will easily out-perform (on your same hardware) these "lightweight" desktops, and GNOME, which all use software rendering to draw the desktop screens.

KDE4 also runs GTK+ applications pretty well (and AFAIK it can accelerate rendering them), and there is now support for keeping the GTK+ theme in step with the chosen KDE4 theme, so that GTK+ applications integrate reasonably well with the rest of the desktop.